A. Field of the Invention
The present invention relates to radiation detection apparatus. More particularly, the invention relates to an apparatus for reducing unwanted infrared radiation from irradiating the detector elements of infrared detection systems, thereby improving the operational performance of such systems.
B. Description of Background Art
The human eye is responsive to electromagnetic radiation in the approximate wavelength range of about 0.4 micrometer (microns) to about 0.75 micron, that wavelength range being referred to as light. Light at the lower and upper ends of the visible spectrum appear violet and deep red, respectively. Hence, electromagnetic radiation in a band of wavelengths ranging from slightly above 0.75 micron to 300 or more microns is referred to as "Infrared" radiation, while electromagnetic radiation in the wavelength range below 0.4 micron to 0.01 micron or less is referred to as "Ultraviolet" radiation.
Detection of infrared radiation serves a wide variety of consumer, industrial, military and other governmental purposes. For example, satellites routinely use infrared detectors to generate images of the earth in infrared wavelengths. Information relative to weather, crop conditions, pollution and mineral deposits can be gleaned from these images.
According to Planck's law, the energy of a photon is given by the product of Planck's constant times the vibration frequency of the photon. The latter frequency is inversely proportional to the wavelength of the photon. Therefore, the energy of an individual photon, called the quantum energy, is less for photons in infrared wavelength regions than for photons in the visible spectrum. For example, a photon in the near infrared wavelength region having a wavelength of 1 micron has one-half the energy of a photon in the middle of the visible spectrum, the latter having a wavelength of 0.5 micron. And, a photon in the "middle" infrared region having a wavelength of 10 microns has a quantum energy one-twentieth that of a 0.5 micron photon.
Many infrared observations are made in the 1-micron to 14-micron wavelength range. This is because many phenomena of interest result in the emission of substantial quantities of infrared energy in this wavelength region. Moreover, there are various "windows," i.e., wavelength regions of relatively high transparency in the atmosphere, in the 1-micron to 14-micron wavelength range.
Operation of infrared detectors, especially in the longer wavelength regions near 10 microns, poses certain problems. First, the low quantum energy of long-wavelength photons, as described above, necessitates rather special detector materials that are sufficiently transparent and responsive to such low energy photons. Typical long-wave detector materials must be cooled to a substantially low temperature to achieve adequate sensitivity, and to reduce thermal noise that is inherent in any detector, to an acceptably low level.
Infrared detectors are often cooled in a device called a dewar. A typical dewar consists of a pair of concentric cylinders; an outer cylinder having a circular window at one end thereof, and an inner cylinder of smaller diameter having an end wall or "cold finger" spaced inwards from the window. Infrared detector elements are mounted on the cold finger. The ends of the two dewar cylinders opposite the window and cold finger are joined together in an annular ring-shaped transition section, thereby forming an elongated annular space between the inner and outer cylinders. This space is evacuated to create a vacuum in the enclosed space between the inner and outer cylinders. Cooling gas or fluid is introduced into the open end of the smaller inner cylinder, thereby cooling the smaller cylinder and attached infrared detector. Conduction of heat between inner and outer cylinders is minimized by maintaining a vacuum between the two cylinders, thereby reducing to a reasonable value the thermal capacity of cooling fluid required to maintain the detector at a particular temperature below ambient.
Another problem associated with operation of infrared detectors at longer wavelengths arises from the inherent nature of infrared radiation. Every object that is at a temperature exceeding that of absolute zero (-273.degree. C.) emits electromagnetic radiation at a rate proportional to the 4th power of the temperature, in accordance with the well-known Stefan-Boltzman law. The wavelength of peak emission of this inherent, "black-body" radiation, is inversely proportional to the temperature, in accordance with Wien displacement law. Thus, the sun, which has a surface temperature of about 6,000 Kelvin (K=.degree.C.+273.degree.) has a peak emission at a wavelength of about 0.56 micron (yellow). On the other hand, an object at "room" temperature (27.degree. C. or 300 K) has a peak black-body emission at a wavelength of about 10 microns.
In many applications, infrared detectors are used to view objects in a scene in which the background surrounding the object, as well as the object itself, in some instances, is at a temperature near 300 K. Thus, it can be readily understood that black-body radiation from the background may interfere with proper operation of the infrared detector in such applications. Accordingly, it is frequently desirable to limit the field of view of an infrared detector, so as to reduce the amount of radiation received by the detector from the background. This reduction in field of view may be achieved by the interposition of a diaphragm plate or aperture stop plate in the optical path between an object viewed and the detector, and/or by surrounding the detector with a baffling arrangement. However, interposition of an aperture plate or baffles in an infrared optical system presents other problems. Since such aperture plates or baffles are often at "room" temperature, i.e., near 300 K, a substantial quantity of black-body infrared radiation may be emitted by these elements towards the infrared detector. This flux of extraneous infrared radiation can increase the electrical noise in the electrical output signals from the infrared detector, thereby decreasing the overall sensitivity and signal-to-noise ratio of the system employing the infrared detector.
In principle, baffles or aperture plates used in an infrared optical system could be thermally coupled to a cooling source used to cool the infrared detector. Thus, some cooled infrared detector systems employ baffles or extraneous radiation-blocking shields incorporated into or attached to the inner cylinder of a dewar. However, cooling other elements in addition to the detector elements in an infrared optical system is oftentimes impractical. Thus, cooling additional elements may require more coolant than is available, in a satellite, for example. Also, thermally insulating baffles and aperture plates, and conducting cooling fluid to them, is often problematical. Therefore, other solutions to the problem of reducing background radiation incident upon an infrared detector have been proposed. Examples of such solutions are contained in the following United States patents:
Gibson, U.S. Pat. No. 2,544,261, Mar. 6, 1951, Infrared Sensitive Cell
Discloses an infrared detector cell having an outer tubular shell and a cooled inner coaxial tubular shell connected thereto forming a closed elongated annular space therebetween, the inner shell having a transverse end spaced longitudinally inwards from the circular end of the large tube containing an infrared window. A detector is mounted in the transverse end of the smaller, inner tube. A mirror surface is applied to the inner surface of the large tube adjacent the detector, to minimize radiation therefrom towards the detector. An indentation for the detector, or a raised annular boss or shield is formed in the end of the inner tube to support the detector to further limit unwanted radiation from reaching the detector.
Jungkman, et al., U.S. Pat. No. 4,914,299, Apr. 3, 1990, Glass Cold Shield
Discloses a shield for a linear array of infrared detectors comprising parallel, longitudinally disposed, infrared-absorbing glass bars.
Wakabayashi, et al., U.S. Pat. No. 4,937,450, Jun. 26, 1990, Infrared Detector Comprising An Evacuated And Cooled Dewar Having An Elliptical Spheroid End Window
Discloses an evacuated and cooled dewar of an infrared detector that has a window end in the shape of an elliptical spheroid, and an IR detection element mounted within the focal circle of the elliptical spheroid. The construction minimizes the sensing of spurious IR rays by the detection element, as well as the heat load for the coolant and the cool-down time.
Japanese Patent, 63-208728, Sep. 30, 1988, Higuchi
Discloses a filter for an infrared detector mounted in a vacuum dewar having a window. The filter comprises a parallel plate having an absorbing thin filter obtained by forming a vapor-deposited optical material which is non-transmissive to out-of-band wavelengths.
The present invention was conceived of to provide versatile means for minimizing unwanted radiant energy from impinging upon an infrared detector, in which certain limitations of prior art techniques are overcome.